If you are on a budget, try eBay - there's a fly spray on the called SHOO, it's made of essential oils - 1 tin of concentrate at £6.50 makes upto 2 litres of fly spray, and it really does work!!
I put it on Lexi last night and she was standing next to friend's horse, friend's was covered in flies, Lexi had 1 or 2 buzzing round her.....